OPTICAL ILLUSIONS.
[Illustration: FIGURE 109.]
[Illustration: FIGURE 110.]
[Illustration: FIGURE 111.]
[Illustration: FIGURE 112.]
The eye is easily deceived and is also very inaccurate in its judgment.
In Figures 109 to 111, all of the lines are of precisely the same
length, yet they appear to differ considerably. The reason for this
error in the estimation of objects is not known. It is especially
noticeable with such objects as a high hat; almost any one will estimate
the height of a silk hat as much greater than it really is. In general,
white objects also appear to be much larger than black objects. This can
be seen by the two inscribed squares one black and the other white in
Figure 112; both of these are of exactly the same size. Probably the
fact that more light reaches the eye from a light-colored object than
from a dark-colored one of equal proportions causes the impression of
greater size.
That the mind has the power of mixing contradictory or conflicting
impressions made upon it is proved by many facts. If a star, Figure 113,
be pinned to the center of a wheel and rapidly revolved, the center of
it will appear jet black; while the outer portions, made up of the
points and the white background, will appear grey which will gradually
fade to a lighter shade from the center black spot outward.
[Illustration: FIGURE 113.]
[Illustration: FIGURE 114.]
If a card be provided with a picture of a bird on one side and a ring on
the other, Figure 114, and this be rapidly spun on one corner, after the
manner of a top, the bird will appear to be in the center of the ring.
This effect is due to the persistence of vision; an image formed upon
the retina requires some time before it can be eliminated so that both
images appear together.
This persistence of vision and the power of suggestion are made use of
in a well-known act which consists of apparently throwing something, for
instance a guinea pig, into the air and causing it to vanish. To perform
this trick, the operator holds the pig in his hand and makes a few
motions suggestive of tossing it into the air. Then, with a final more
extreme motion on the downward swing of his hand, he drops the pig and
swiftly moves his empty hand upward. Due to the persistence of vision,
the audience actually still sees the pig and due to the suggestion of
tossing the eyes look upward, and thus the persistence of vision and the
power of suggestion create the illusion.
[Illustration: FIGURE 115.]
_Dissolving Views of Living Pictures._--This act requires a large glass
plate of good clear quality arranged upon the stage, as shown in Figure
115. _One_ of the poses is arranged as at the black circle behind the
glass and when illuminated is seen by the audience. The other is
arranged at one side. The lighting of both is connected to the same
dimmers in such a manner that when the light of one is increasing, that
of the other is diminishing. Thus the two figures are dissolved into one
another. The paths of the various rays of light to different parts of
the auditorium are shown in the drawing and it is possible to procure
perfect registry.
_Human Figure Floating or Performing in Air._--This act is arranged by
means of a large mirror placed upon the stage as shown in Figure 116.
In the _pit_ out of sight of the audience is a revolving table draped in
dull black and all of the pit is draped in the same manner. A figure
lying upon this table dressed in light clothes will appear erect in the
mirror as indicated by the arrow. If now the table is revolved, the
figure will be seen as turning over in the mirror. The figure upon the
table may perform a number of evolutions suggestive of floating, flying,
etc. The black cloth of the pit will reflect no light and only the
figure will be visible. If a glass plate is used instead of a mirror,
suitable scenery may be arranged back of it. The figure must of course
be brightly illuminated. By moving the table upon which the figure rests
across the pit, the figure will appear to move along.
[Illustration: FIGURE 116.]
_Head Suspended in Air._--This act is worked out in bright light. It
requires a mirror with a hole in the center of it large enough to admit
the head of a person, as shown in Figure 117. Above the mirror suitable
decorations are provided which cause one looking at the mirror to
imagine seeing the back wall of a room. The ceiling must of course be
kept out of view as much as possible. A person looking on sees the head
and the mirror gives him the impression of vacant space about it. The
head must be surrounded by a collar or drapery of some kind so that no
reflection of it will be visible.
[Illustration: FIGURE 117.]
[Illustration: FIGURE 118.]
_Magic Cabinet._--In this act a person enters the cabinet, Figure 118.
The outer doors are closed for an instant and then opened. The person
has disappeared! The disappearance is brought about by pulling the two
mirrors, arranged inside of the cabinet, into the position indicated by
the dotted lines. When the cabinet is open for inspection, these are
swung to the side and are invisible. The backs are of the same design as
the rest of the interior cabinet. When they are pulled together and hide
the person behind them, they reflect the side walls of the cabinet and
are not noticed. If the cabinet is well made and the act skilfully
performed, the outer doors may even be omitted.
_Head Resting on Table._--For this act a hole large enough to allow a
person’s head to project through is cut in the top of a table, Figure
119. This table has only two legs and is fitted up with mirrors, as
indicated by shading. The reflection of the two legs in the mirror
causes the observer to see four and imagine that the space below the
table is vacant.
[Illustration: FIGURE 119.]
_Multiplication of Images._--If three large mirrors be arranged in the
form of an equilateral triangle, as indicated in Figure 120, a person
standing in the center will see his image reflected so many times that
he will receive the impression of being in the midst of a crowd. He will
see the reflections of reflections repeated until by absorption so much
light is lost that they finally become invisible.
[Illustration: FIGURE 120.]
[Illustration: FIGURE 121.]
_Trick Mirror._--Rather startling effects can be produced by a thinly
coated mirror _A_, Figure 121, behind which an electric light is
arranged so that it may be easily turned on. The space behind the
mirror being dark, no one suspects that it is anything but a common
looking glass. It is, however, quite transparent when the light behind
it is turned on. The person in front viewing himself may thus suddenly
be brought to see anything that is behind the mirror. If another set of
mirrors is arranged, as shown by shaded lines, the person in front of
the mirror may suddenly be made to see a head floating in the air in
front of him.
[Illustration: FIGURE 122.]
The manner in which one can see through a brick is illustrated in Figure
122. Four mirrors reflect the light around it. If the mirrors are
properly arranged, the person will imagine he is looking through the
brick.
[Illustration: FIGURE 123.]
The face of a person may be thrown upon a screen among clouds or other
pictures in the manner shown in Figure 123. The face occupies the space
marked by a half circle and two arc lamps with condensers are trained
upon it at very close range. The face itself acts as a reflector
projecting itself through the object lens in the center. This act is
very trying on the person whose face is reflected.
At a certain distance from a concave mirror, an object will be seen
inverted. If the mirror be brought closer, the image will gradually go
out of focus and, by bringing the mirror still closer, it will gradually
come into focus again but this time erect. Properly arranged such a
mirror can be made to give weird effects, for instance a skeleton,
rushing at the observer; and the image will appear to step out of the
mirror.